Bone conduction hearing aids are prescribed to patients who cannot use conventional air conduction hearing aids because of chronic ear infection or a congenital or acquired deformity of the outer and/or middle ear. Sound or vibration generating transducers are used as speakers in such bone conduction hearing aids. Sometimes such transducers are referred to as a bone conduction transducer.
A traditional bone conduction hearing aid consists of a bone conduction transducer contained in a plastic casing which is pressed with a constant pressure of 3-5 Newton against the skin over the bone behind the ear. Microphone, amplifier and battery are placed in a separate enclosure at a safe distance from the transducer to avoid feedback problems. The most significant disadvantages with this type of bone conduction heaing aid are that it is uncomfortable to wear because of the constant pressure against the skin and that the soft skin over the skull impairs the transmission of the vibrations from the transducer to the bone.
Since the early 1980s another type of bone conduction hearing aid was introduced—the bone-anchored hearing aid (BAHA)—where the bone conduction transducer is attached directly to the bone using a skin penetrating bone-anchored titanium implant, e.g. SE8107161, SE9404188 or Tjellström et al. 2001. In this way a bone conduction hearing device could be designed where all components are capsulated in a single housing. This device also offers higher gain and an improved wearing comfort. To improve the BAHA system performance further, a new type of bone conduction transducer was developed called Balanced Electromagnetic Separation Transducer (BEST) which is described in patents U.S. Pat. Nos. 6,751,334, 7,471,801; SE0666843 and H{dot over (a)}kansson 2003.
A new generation of bone conduction devices is under development in which a capsuled BEST transducer is completely implanted in the temporal bone and thus the skin and soft tissue can be intact. Both the signal and the energy are here transmitted through the intact skin using an inductive coupling arrangement, as described by H{dot over (a)}kansson et al. 2008 and 2010. The benefits of implanting the transducer in the temporal bone, compared with a transducer that is externally worn, are many. Most importantly the permanent skin penetration is not needed which otherwise require daily care and in some cases it suffer from infections and possibly also the entire implant can be lost as a result of such complications. In addition, increased vibration sensitivity is also obtained as the implanted transducer, for anatomical reasons, preferably is placed in the temporal bone closer to the cochlea (H{dot over (a)}kansson et al. 2010). Finally, the size of the externally worn sound processor will be smaller (as it do not need to contain the transducer) and the stability margins are improved.
For obvious reasons, it is of utmost importance for a bone conduction transducer in general and implantable transducers in particular to have a high mechanical vibration/sound output, high efficiency, and have a small size. For an implanted transducer where a replacement requires a surgical procedure it is perhaps even more important that the reliability of the transducer is very high and proper function should preferably be life-long. These demands require new solutions as the transducers with today's technology have limitations and shortcomings in most of these respects. Transducers with current technology are too large and may not fit in a large proportion of temporal bones especially in patients with history of the ear infection where the temporal bone has a tendency to significantly deform and shrink in size. It is also widely known that the transducer is the most vulnerable component in today's bone conduction hearing aids. Above all, it is the small and vital air gaps in the transducer that are the main source of these reliability problems.
The primary objective of the present invention is to minimize the BEST transducer in size by means of a new topology without sacrificing vibration output performance. A second objective is to find a design where the air gaps can easily be inspected to ensure the quality of the transducer.
Other applications for bone conduction transducers in addition to hearing aids are for example in communication applications, audiometric testing applications and in vibration testing equipment. The present invention is equally applicable in such applications.